A number of consortiums have been formed to develop satellite based Personal Communications Systems (PCS) with global coverage. These systems include Globalstar (Globalstar System Application before the FCC by Loral Cellular Systems, Corp., Jun. 3, 1991), Odyssey (Application of TRW Inc. before the FCC to Construct a New Communications Satellite System "Odyssey," May 31, 1991), Ellipso (filed November, 1990), and ECCO (filed by Constellation Communications Inc. in June, 1991), among others. All of these systems employ multibeam antennas which provide isolation between geographical areas, thus allowing spectrum reuse.
The intent of these systems is that a subscriber can place telephone calls directly through the satellite network from almost anywhere on the Earth, using a portable handset much like the present cellular telephones. These systems also include mobile and fixed user terminals. All of the systems mentioned are required to use spread spectrum CDMA techniques for bandsharing purposes.
In 1990, Gilhousen et al published the paper "Increased Capacity Using CDMA for Mobile Satellite Communication"(IEEE Journal on Selected Areas in Communications, May 1990) which showed that system capacity is increased in a multibeam satellite system by using CDMA with a frequency reuse factor of one (i.e. the total frequency band is reused in each beam) compared to orthogonal systems such as FDMA or TDMA, which would typically employ a frequency reuse of 1:3 or 1:5, depending on the amount of isolation between beams.
The use of "marginal isolation" to provide full frequency reuse in quasi-orthogonal (nonsynchronous) CDMA system is suggested in U.S. Pat. No. 4,901,307 by Gilhousen, Jacobs and weaver (Feb. 13, 1990).
The Globalstar application discloses a signal which is essentially the same as the IS-95 terrestrial cellular standard. This system uses Orthogonal CDMA (OCDMA) on the forward (base-to-mobile) links and nonsynchronous CDMA on the return links. However, no distinction is made between the OCDMA forward link and the nonorthogonal CDMA return link in frequency reuse. Full frequency reuse is employed in each beam on both forward and return links, and this is assumed to be the best choice.
The basic signal format for OCDMA is disclosed by M. J. E. Golay in IDA Report 108, page 110 (1965). The system described is orthogonal in both the forward and return link directions.
OCDMA forward and return links are being considered for satellite PCS systems. Related patents which disclose improvements include U.S. Pat. No. 5,375,140 to Natali, and titled "Doubly Orthogonal Code and Frequency Division Multiple Access Communication System," and U.S. Pat. No. 5,668,795 to Magill et al and titled "Modulation System for Spread Spectrum CDMA Communication, incorporated herein by reference.
The capacity of a synchronous OCDMA system can be increased, under certain conditions, by employing a frequency reuse factor other than one and taking advantage of the orthogonal properties of the multiple access codes within a beam as disclosed in our patent application Ser. No. 08/989,466 filed Dec. 12, 1997 and entitled "Increased Capacity in an OCDMA System by Frequency Isolation". For example, the total capacity of the OCDMA system may be increased by employing 1:3 frequency reuse rather than full frequency reuse in every beam. In this case, frequency isolation is more important than additional spectrum to maximize capacity, even though full frequency reuse is possible.
Typically, for 1:3 frequency reuse, the available spectrum would be split into 3 subbands, which are assigned to the beams in a 3-frequency reuse pattern. Unfortunately, for the same forward link power, the power flux density from the satellite has now tripled in the one-third of the band used. This is disadvantageous since, in many cases, the system capacity becomes limited by the FCC/CTI limitation on maximum power flux density of the satellite forward link signal incident on the Earth's surface. It also changes the band sharing scenario, since the signal now takes up only 1/3 of the available spectrum in each beam, but is potentially 5 dB stronger.